Before his death, Luther claimed that The Bondage of the Will and the Catechisms were the only things that he had written that were worth reading. I think I have already article in my mind about how one should read them in light of one another, but that would be a much larger post. What I'm interested in focusing on here is Luther's constant refrain in the discussion of the Ten Commandments "We should fear and love so that..." How should we take this? If we follow Luther's own words as a hermeneutical key (namely that the BOW and the Catechism are his best works and therefore a definitive representation of his thinking), "fearing and loving" should be understood in terms of Luther's own dialectic of the hidden and revealed God.
For those unfamiliar, in BOW Luther speaks of God preached and God not preached. If we look at creation as a whole as a sphere of God's activity, the logic of God's action will appears incomprehensible to us. Whereas God in his revelation in Word and sacrament states "I will not delight in the death of the sinner," God insofar as he works all things certainly does work death to sinners. He of course does this for good reason: All are born with original sin. The difficulty is that to some, through his electing will, God approaches through Word and sacrament, and converts, justifies, and sanctifies them. Others (who are of course no less sinful), he does not work faith and works their destruction. God therefore works within his creation through many divine masks (larva Dei), through some he redeems, through others he destroys. There is no "thinking into" revelation (as in many Neo-Platonically inspired Christian theologies: Augustine, Barth, Aquinas, Calvin, etc.) to see why this is the case and not another situation. God is not just incomprehensible, but actively hides behind his mask and (to use Forde's phrase) "shuffles" them at will. This reality is a natural outgrowth of the dialectic of law and gospel: In some masks, God comes to us as law and in others as gospel. Since the law and the gospel are actually opposites, there is no "thinking into" them. Both are the will of God, but we actually can't see how they are internally coordinated in God's mind. The best we can do is to see from the perspective of faith how the law drove us to the gospel and how Christ has fulfilled the law on our behalf. Nevertheless, these are not realities latent in the law itself and so the mystery of the divine hiddeness remains.
Within this situation, what is the Christian to do? Luther tells us that the revealed God of the gospel, that is, the God of grace, is God's real self (despite what might be considered evidence to the contrary!). When we approach God hidden, or God under his masks of law, we can only find condemnation- something actually alien to God in his proper nature (opus alienum). Consequently, we should flee from the God of hiddenness and wrath, to the God of grace, that is, from God not preached to God preached. Nevertheless, if both are God, how do we know that God preached is the more authentic of the two? In the Galatians commentary, Luther talks about God in his hiddenness and wrath condemning and destroying Jesus who bears the sins of the world. The law (in a sense) tries to destroy the promise by condemning Jesus who has entered into solidarity with those under the God of hiddenness and wrath. In spite of this, Jesus atones for sin, undoes the power of the law, and reveal God's true heart. Since Jesus (the revealed God of grace) has gone up against the hidden God of wrath and law, and come out the other side victorious, those who are united with him by faith can also share in his victory and therefore have nothing to fear from God not preached.
Faith clings to the revealed God against the hidden God, and therefore the shape of the Christian life of trust is fleeing from one to the other. This can be observed throughout the history of salvation. With Adam and Eve, God establishes his relationship with them through two trees- the tree of life and the tree of the knowledge of good and evil. God attaches his promise to all the trees of the garden as means of mediating the good to them ("you may eat..."). He gives them the tree of life as a sacrament of immortality. Nonetheless, he also establish the tree of the knowledge of good and evil as an alternative to the means of his grace. This was not a test (Wesley, John of Damascus), or a means of earning their glorification (Reformed Federal theology), but rather a manifestation of the irrationality of God not preached. In other words, the tree is in a sense inexplicable. Why put the possibility of becoming evil in the midst of the good creation? Such is a mystery, a manifestation of the hidden God. Nevertheless, it was also formative of the obedience of Adam and Eve, which ultimately constituted a sacrifice of praise to God (Luther). Finding God not preached through the tree of the knowledge of good and evil, structure of their believing existence was the fleeing from God not preached (that is, God of irrational, destructive condemnation) to God preached, i.e., his manifestation in the other trees of the garden and the tree of life. They only enter into sin and condemnation when they sought God not preached and ignore God as his was manifested in his Word to them.
We can of course name other examples of this: Jacob is attacked by the hidden God by the river, but seeks the name and therefore self-donating gift of the preached God who had already covenanted with him. Moses is attacked by God unpreached (for no apparent reason) on his return to Egypt, but his wife circumcises their son, and the attack is ended when God preached (that is, the God of the gospel manifested in the promise connected to circumcision) is sought. Lastly, God unpreached is encountered on Sinai (much to the terror of Israel), but he establishes himself later as God preached in the Tabernacle and later on Mt. Zion as the preached God who (as John Kleinig has shown) sacramentally channels his alien holiness to the people.
Turning to the Catechisms, I would argue that when Luther speaks of "fearing and loving" God he is talking precisely about this fleeing from one God to another. God not preached cannot be trusted, and does nothing but promote unbelief in his goodness by his irrational and terrifying presence. And so one must seek God where he has given his Word and promised to be gracious. Perfect faith means perfect obedience to God's Word because it means that we trust in what God is doing in his different masks that he has attached a word to. This is why Luther states in his discussion of the first commandment that it is both the gospel (we need no other gods than God, because he is supremely trustworthy) and a summary of the law. In trusting God as a gracious God, we trust what he says about all his creatures and the goods which he will give to us through them. We trust that God has put parents and other authorities above us for our good. We trust that God has channeled certain goods connected with our sexuality through marriage, and consequently we don't need to seek them elsewhere. Finally, we trust that God will ultimately take care of us and so we don't need to covet, lie, or steal. Hence, all the commandments demand faith in God, and each commandment is merely an illustration of what trusting in God looks like and what it does not look like. If one looks to God not preached and away from God's promises to channel the good through his creatures that he makes in Holy Scripture, one will of course become terrified and unbelieving. One will seek the good autonomously, apart from God's promises and consequently look for it in the wrong sources. One will grasp at it, because this will be the only means of securing it. One will in effect make themselves their own god through self-trust. Hence, we must flee from God not preached ("fear") found in the inappropriate means of gaining the good, and cling to God preached ("love") in order to believe God's commandments.